Automobiles having leaf-spring suspension systems are manufactured with the rear wheels aligned in a fixed position. This is acceptable as long as the alignment of a particular vehicle complies with the specifications established by the manufacturer. However, rear-wheel misalignment not conforming to the manufacturers' specifications may occur either upon production or subsequent to production as the result of an accident. Rear-wheel misalignment compromises the performance of the vehicle. For instance, misalignment causes undue and uneven wear on the automobile's tires, unsatisfactory handling, and stress on components such as the rear wheel axle assembly.
Prior devices have not been developed specifically for correcting rear-wheel misalignment in sport or utility automobiles. However, several inventions have been patented for mounting or balancing the load on suspension systems. For instance, Marian's U.S. Pat. No. 3,730,508 describes a device for mounting the axles of a vehicle to a vehicle's suspension system. The device allows movement of the axle to properly balance a load placed on the leaf spring. The device comprises an upper plate 24, a spacer block 28, an adjustment block 46, and a support block 56. Of these elements, spacer block 28 and adjustment block 46 are in mating face-to-face contact with each other, as are adjustment block 46 and support block 56. These are the only two pairs of mating plates or blocks described. Both pairs have mating surfaces that bearingly interengage one another, allowing discrete adjustment, but preventing infinite adjustability. This is because the support block includes a projecting lug that fits in one of a plurality of recesses provided at specific intervals in the spacer block, and because the adjustment block and support block have engagable teeth. Marian's device was designed such that the blocks and plates matingly interlock with each other to only allow specific, incremental axle adjustments that are determined by the distance between the recesses. Moreover, the spacer block increases the ride-height of a vehicle, which is aesthetically unacceptable.
Traylor's U.S. Pat. No. 3,645,555 describes a tandem axle suspension system designed for heavy-duty highway transport vehicles. This invention employs two plates that are attached to the axle and leaf spring of the transport vehicle. The plates have serrated faces that allow only specific, incremental adjustments of the axle position along the spring axis. Furthermore, as with Marian's device, axle adjustments can not be achieved while the device supports the weight of the automobile. The adjustment must be tested while the suspensions system bears the weight of the automobile. If the adjustment does not achieve the desired correction, the weight of the automobile again must be removed from the suspension system and a new adjustment tried.
Similarly, Hertrick's U.S. Pat. No. 5,046,756 and Australian patent No. AU-B-56924/90 describe a device that is designed for long-haul heavy vehicles having a rigid or beam-type drive axle assembly supported by a compound leaf-spring assembly. Hertrick's invention comprises a pair of opposed plates that are bearingly interengagable. One of these plates is secured against axial movement relative to the vehicle's suspension assembly, and the other plate is secured against axial movement relative to a wheel axle. Each of the plates includes a surface designed to matingly interlock with the opposed plate to prevent movement relative to each other when bearingly interengaged. Preferred embodiments of the bearing plates include complementary corrugations oriented transversely to the direction of relative movement.
As with the Marian and Traylor inventions, Hertrick's invention permits only specific, predetermined incremental plate adjustments in a direction transverse to the orientation of the corrugations, i.e., along the longitudinal axis of the vehicle. If the preferred alignment position occurs at a point intermediate the corrugation recesses, then the preferred alignment position can not be achieved. Furthermore, as with the Marian and Traylor patents, if an adjustment is incorrect, Hertrick's two plates are interlocked with each other and cannot be adjusted relative to each other while the suspension system bears the weight of the automobile. Therefore, the weight of the automobile must be removed from the suspension system and a new adjustment tried.
To align the rear wheels of an automobile using Hertrick's device, it is first necessary to calculate the actual angle at which the axle is installed. The actual angle is then compared with the optimum angle to determine the angular misalignment. The angular misalignment must then be converted to a linear distance using a geometric formula. The opposed plates are then positioned between the leaf-spring suspension assembly and the differential housing. The linear misalignment distance is then corrected, if possible, by moving the plates relative to each other in increments corresponding to the specific, predetermined distances provided by the corrugated opposed plates. If the misalignment condition is not calculated correctly, then the whole assembly must be disassembled to correct for the miscalculation.
Hertrick's invention was designed solely for use with long-haul trucks having a resilient block, which is shown as element 33 in FIG. 3 of Hertrick's patent. Thus, to use Hertrick's device, an automobile must include the rubber block 33 when the automobile is purchased. Alternatively, the automobile's differential housing bracket must be removed and the resilient block installed between the axle and the adjustment device. Virtually none of the automobiles currently being sold include a resilient rubber block. Therefore, to use Hertrick's device requires that the differential housing bracket be removed and a rubber resilient block attached to the differential housing prior to adjusting the suspension system. Introducing the resilient block required by Hertrick substantially increases the ride height of the automobile. Furthermore, the resilient block requires using an attachment bracket. This bracket, which extends from and interrupts the bottom surface of the lower plate, allows the plate to be attached to the resilient block.
Preferred devices for correcting a misalignment must resist the transverse movement of the automobile. Hertrick's device includes only screws that prevent such movement. These screws (element 92 in Hertrick) are set in recessed regions that are machined in the top plate. These recessed regions do not extend upwardly from this plate, and therefore likely do not provide adequate resistance to the transverse shearing motions that occur with an automobile.
For the reasons stated above, Hertrick's invention is labor intensive. Moreover, Hertrick's invention does not cure all the problems associated with curing a misalignment. For instance, Hertrick's invention provides only specific, incremental adjustment capability. Hertrick's device does not allow infinite adjustment capability along the area of contact between the plates comprising the device, and therefore cannot achieve the optimal position for curing the misalignment.
McGee's U.S. Pat. No. 3,674,246 describes a device that allows an axle and spring beam of an automobile to be adjustably positioned relative to each other. The device comprises a rectangular adjustment plate that rests on an upper leaf of the spring beam, and an arcuate saddle block that accommodates an automobile's axle. The device allows adjustable positioning of the axle relative to the beam members. McGee's device requires that the saddle block be permanently secured to the axle to prevent transverse movement of the axle relative to the beam. Hence, repositioning the axle, which may be necessary after an accident or if the optimum alignment is not obtained on the first try, is difficult and requires severing the weld between the axle and saddle block. Severing the weld is not only inefficient, but may damage the axle or the saddle block.
All the inventions discussed above provide inadequate means for accommodating the substantial transverse shearing forces, especially between matingly-engagable adjustment plates, that are generated by automobiles. Alternatively, the inventions discussed above require inefficient and permanent alterations to the automobile, making subsequent adjustments difficult. Both the Marian and Traylor inventions provide only opposed, bearingly-engagable plates joined by U-bolts that extend through the leaf-spring assembly. Hertrick's invention uses screws that extend through the top plate and into a tapped region of the bottom plate. The length of these screws is limited by ride-height considerations, whereas increased screw-diameter necessarily weakens the top and bottom plates that must accommodate the screws. McGee's invention permanently secures the saddle block to the axle.